The present invention generally relates to a method for fabricating a reflective-type liquid crystal display panel and panels made and more particularly, relates to a method for fabricating a reflective-type liquid crystal display panel by utilizing a reflector equipped with an inclined surface such that reflected noise signals and image signals are separated to improve the noise/signal ratio and panels fabricated by such method.
Liquid crystal display devices have been used for many years. In the beginning, their uses have been concentrated in small appliance applications such as electronic watches and calculators. LCD's are now used in applications for instrument panel numerical displays and graphical displays. Advantages presented by LCD's are their inherent properties of small thickness, lightweight, low driving voltage required and low power consumption. As a consequence, more recent applications of color LCD's can be found in small screen television sets, notebook computer display panels and video camera view finders as replacements for conventional CRT's.
A liquid crystal display device can be made either a color unit or a black and white unit. The device may also be constructed as a reflective-type or as a transmittive type, depending on the light source used. Since liquid crystal molecules respond to an externally applied electrical voltage, liquid crystals can be used as an optical switch or as a light valve. A typical liquid crystal display cell arrangement is shown in FIGS 1A and 1B.
Referring initially to FIG 1A, wherein a liquid crystal display device 10 is shown. Liquid crystal display cell 10 is a single pixel which is constructed by two parallel glass plates, i.e., an upper plate 12 and a lower plate 14. Both the upper plate 12 and the lower plate 14 have a polarizing film 36 and 32 coated on its outer surface. The cavity 18 formed between the two plates 12 and 14 is filled with a liquid crystal material 20. One of the most commonly used liquid crystal material is of the twisted nematic (TN) type wherein the term twist refers to the tendency of the liquid crystal to form chains that rotate from one side 22 of the gap between the plates to the other side 24 of the gap. The degree of rotation can be controlled during the fabrication process.
As shown in FIG. 1A, light beam 28 passes through the polarizer 36 and then through the liquid crystal display cell 10 having its polarization direction rotated by following the physical rotation of the liquid crystal molecules 26. As shown in this simplified illustration, the polarizer 32 on the exit side 24 of the liquid crystal cell 10 is positioned such that it allows a rotated light beam 30 to pass through the polarizer 32. When viewed from the side of the polarizer 32, the pixel or the liquid crystal cell 10 thus appears clear, i.e., in a transmitting mode.
A transparent electrical conductor (not shown) such as indium-tin-oxide (ITO) is normally deposited on the inner surfaces of the glass plates 12 and 14. The transparent electrical conductor layer is patterned into a series of mutually perpendicular lines (not shown). When a voltage is applied across the cell cavity 18 by addressing the appropriate line on each side of the cell, the liquid crystal molecules 26 reorient themselves to follow the applied electric field. The liquid crystal materials 26 are thus untwisted as shown in FIG. 1B. The passage of the untwisted light beam 34 is blocked by the exit polarizer 32 as long as the voltage is present. When the voltage is turned off (shown in FIG. 1A), the liquid crystal molecules 26 returns to their original state and the cell or the pixel becomes clear again. As previously stated, typical voltages and currents required to activate the liquid crystal molecules are relatively low making it an ideal candidate for incorporation in battery-operated equipment where a low power consumption is essential. A typical twisted nematic (TN) liquid crystal cell used for small displays have a twist angle of 90.degree.. More recently developed supertwisted nematic (STN) material forms a twist angle up to 270.degree. and thus allow higher contrast so that many pixel elements can be multiplexed in a single display.
While the liquid crystal display device 10 shown in FIGS 1A and 1B is the transmittive type, liquid crystal display devices of the reflective-type are also used. In a reflective-type liquid crystal display device, one of the upper plate 12 and the lower plate 14 (shown in FIG. 1A) is replaced by a reflector plate which is light reflective and not transmittive. The reflector plate may be fabricated of a glass substrate with transistors or other active components built on top and coated with a metal reflective layer. In the reflective-type liquid crystal display device, the light source for illuminating the liquid crystal display is from the ambient such that a display is viewed in a reflective manner.
A drawback of the reflective-type liquid crystal display device is the noise signals reflected from the top, or the cover glass plate of the display device. In a conventional reflective-type liquid crystal display device, the reflector plate and the top cover plate are parallel to each other. When an outside light source is used to produce an image in the liquid crystal device under the reflective principal, the light reflected from the reflector plate and from the top plate have the same reflective angle. Since the light reflected from the top cover plate does not produce the image formed in the liquid crystal display, only noise signals are produced which decrease the contrast of the display device. Furthermore, the noise/signal ratio of the device is also increased which affects the quality of images produced by the display device.
For instance, as shown in FIG. 2, a conventional reflective liquid crystal display device 40 consists of a top plate 42 and a bottom plate 44 which has a reflective coating 46 deposited thereon. On the top plate 42, an incident light beam 48 is directed at the top surface 50 of the top plate 42. The incident light beam 48, at intersecting the top surface 50 of the top plate 42, deflects into a light beam 52 and reflects into a light beam 54 simultaneously. The deflected beam 52 penetrates through a liquid crystal medium 58 and is reflected by the reflector surface 46 into reflected beam 56. The reflected beam 56 is then deflected by the top plate 42 into light beam 60. The deflected light beam 60 carries the image formed in the liquid crystal device 40 while the deflected light beam 54 does not carry such image and therefore is treated as a noise signal. To a human observer 62, the noise signal 54 interferes with the liquid crystal display signal 60 and therefore decreases the contrast of the image produced. Furthermore, the noise signal 54 increases the noise/signal ratio of the liquid crystal device 40.
It is therefore an object of the present invention to provide a reflective-type liquid crystal display device that does not have the drawbacks or short comings of the conventional devices.
It is another object of the present invention to provide a method for fabricating a reflector for use in a reflective liquid crystal display device by using an inclined reflector surface.
It is a further object of the present invention to provide a method for fabricating a reflector for use in a reflective liquid crystal display device that has an inclined reflector surface formed by a photoresist layer.
It is another further object of the present invention to provide a method for fabricating a reflector for use in a reflective-type liquid crystal display panel that has an inclined surface by depositing a layer of a positive photoresist material onto the reflector surface and then imaging the layer with multiple dosages of energy such that a multi-step surface is formed.
It is still another object of the present invention to provide a method for fabricating a reflective-type liquid crystal display device with a reflector that has an inclined surface of at least 0.5.degree. as measured from a longitudinal axis.
It is yet another object of the present invention to provide a method for fabricating a reflective-type liquid crystal display device with a reflector that has an inclined surface by first providing a reflector that has an inclined angle of at least 0.5.degree. as measured from a longitudinal axis and then filling the device with a liquid crystal material between the reflector and a light transmittive panel.
It is still another further object of the present invention to provide a reflective-type liquid crystal display device that includes a reflector surface positioned at an inclined angle of at least 0.5.degree. when measured from a longitudinal axis.
It is yet another further object of the present invention to provide a reflective-type liquid crystal display device that has a positive photoresist layer formed on a reflector plate such that an inclined surface with an angle of at least 0.5.degree. as measured from a longitudinal axis is employed.